Abstract Energy–exergy analysis and parameter design optimization of the KCS-11 solar system with an auxiliary superheater are studied in low-grade thermal energy conversion (LTEC). Firstly, from a thermodynamics point of view, the corresponding calculation model is built to solve the system state points as well as the exergy input/output/loss for each system component. And then, according to the characteristics of the KCS-11 solar system, the verification items are given to verify the correctness of the calculation model. Afterward the model is proved to be correct by sampling check a set of calculation data. On that basis, the corresponding parameter design optimization and system performance analysis are carried out from the viewpoint of the maximization of the exergy output in KCS-11 solar system at a certain scale. Results show that the mass flow rates of working fluid and solar collector subcycle and also ammonia mass fraction are important system operation parameters that should be optimized to deduce the irreversible behavior of the solar system for producing more useful energy. Meanwhile, the heat-transfer rate distribution ratio of the superheater should be large enough to ensure that the expanding vapor in the turbine is superheated. Finally, an optimization calculation case is designed for illustration by using the monthly mean solar radiation statistics in Kumejima Island of Japan. In this case, the maximum generated power is 491 kW showing 35.6% exergy efficiency and 6.48% energy efficiency of the system for the month of August. The size of the system in terms of power generated of each major equipment is listed as follows: solar evaporator (370 kW), superheater (106 kW), condenser (298 kW), turbine (491 kW), separator (43 kW), absorber (37 kW), pump (8 kW), regenerator (38 kW), and diffuser (17 kW). And the main system exergy losses are associated with internal consumptions of exergy in turbine (92 kW) and condenser (97 kW) due to irreversibilities. In this way, the maximum annual power generation of the KCS-11 solar system is about 553,520 kW h.